There will be probes. There will be more space telescopes. There will be new questions asked and answered about black holes, red dwarfs, white dwarfs and whether the Universe can stop expanding.

But, in the absence of a superpower space race, only one question matters enough to mankind to justify the expense and risk of renewed human exploration of the cosmos: “Is there anybody out there?”

This is not about little green men (although in a universe with perhaps ten million billion planets no one should rule them out). It's about the nature of the Universe and our place in it.

Scientists are looking for answers in three ways: some, neglected by the US federal government but funded by tycoons such as Microsoft’s co-founder, Paul Allen, have formed the Search for Extra-Terrestrial Intelligence (SETI) and are listening out for it with an international network of radio telescopes.

Others are peering at those ten million billion planets with ever more sophisticated optical telescopes to see if any of them resemble Earth. And Nasa, with help from 13 other space agencies, is going to Mars.

The first US President to pledge to put mankind on the red planet was George H. W. Bush. His son has repeated that undertaking with the extra challenge of returning to the Moon first. Many view this as an unnecessary distraction; some believe that the real reason is to get there before China, which has said it wants to put a man on the Moon by 2020.

But few dispute that a permanent lunar base could serve as a useful test bed for long-duration spaceflight – Mars is at least a two-year return trip – and the more evidence that emerges of potentially life-sustaining conditions on Mars, the stronger the argument for sending people, not just robots.

As Professor Martin Barstow of the University of Leicester puts it: “In the end, human exploration of Mars is inevitable. A human can pick a rock up, turn it over, see something green and say ‘Wow!’ It’s very hard to set up a robot to do that.”

By most estimates it will be at least 20 years before a person walks on Mars, and he or she will probably blast off aboard a conventional-looking, one-use-only Ares V rocket. (When the last space shuttle retires in 2010, so will the concept of a reusable space vehicle, except for space tourists lunging to the edge of Earth’s atmosphere for a few minutes’ of weightlessness.)

This means that humans’ top speed through space will remain for the time being what it was for Yuri Gagarin – about 17,000mph. Visionaries are busy dragging concepts such as nuclear rockets and solar sails out of science fiction and on to the drawing board, but it could be generations before they can propel real spacecraft.

Robotic probes are studying the moons of Saturn (Cassini), and heading for Pluto (New Horizons, due to arrive there in 2014) and Mercury (first fly-by, January 2008). But the furthest man-made object from Earth, Voyager 1, is 30 years into its journey and only just leaving the solar system. If space is the “new ocean”, as President Kennedy suggested, we have barely left the continental shelf.